Research On DME Flash Boiling Spray Characteristics And Its Application On Diesel Fuel HCCI Engine

Due to energy crisis and environmental protection consciousness, exploring alternative fuel and high efficient utilization the conventional fuel become the main researching direction of modern internal combustion engine (ICE). Recently, Dimethly Ether (DME) as an excellent alternative fuel has been attracted by many researchers owning to its good spray characteristics and ignition. In order to realize the high thermal efficient with low emissions for ICE, homogeneous charge compression ignition (HCCI) engine has been attracted attention by many ICE researchers in the world. It is well known that pure diesel fuel is very difficult to prepare homogenous mixture for pure diesel fuel due to its high viscosity and low saturated vapor pressure. On the other hand, pure DME fuel will cause the dysfunction of a conventional film bearing between the needle and sleeve of the injector, and result in leakage and wear due to its very low viscosity. It is a good way to solve above the problems using DME/diesel blended fuel. Therefore, the purpose of this paper is to study the DME flash boiling spray characteristics and its application on diesel HCCI combustion engine. The main contents and achievements of this research are as following:In order to understand the detail information of flash boiling spray phenomenon, so after reading a great number of literatures on relevant research field, this paper introduces the basic principles of flash boiling spray and different characteristics between the flash boiling spray and traditional spray, and shows that flash boiling spray is a novel way to make the homogenous mixture for HCCI engine. In order to simulation the flash boiling spray process, a macro-phenomenal flash boiling spray model was set up with KH-RT breakup model according to the mechanism of flash boiling spray. In order to simplify the model and improve the computational efficiency, this flash boiling model mainly considers the effects of superheat on the spray cone angle, spray penetration and the rate of vaporization and temperature, which ignores the bubbles inside the superheat droplets.In order to solve the limitation for modeling flash boiling spray of KIVA-3V2 code, the phenomenon flash boiling spray model is coupled with KIVA-3V2 code successfully. The results of experiment and calculation are basically identical. Under the same spray conditions, the results show that the spray tip penetration of DME was shorter than that of the diesel fuel, but the spray cone angle of DME is larger, it was also found that there was a obvious vortex ring on the each side of the head of the DME flash boiling spray, and its concentration field is also better than diesel spray. The SMD also is smaller than diesel spray.The spray characteristics of pure diesel fuel, pure DME fuel and DME/diesel blended fuels with DME mass fraction of 25%, 50% were investigated by a laser size analyzer (LSA) and a digital camera. In order to simulate DME/diesel blended fuel spray characteristics, the thermal physical parameters databases are also added into KIVA-3V2. The experimental results agreed with the numerical results very well. The results reveal that the DME concentration in the DME/diesel blended fuels had significant influence on both the macroscopic and microscopic of the spray due to the micro explosion and flash boiling function. The spray cone angle of blended fuel was increased with the increase of DME concentration. At the centerline of the spray, the SMD decreased with the increase of the DME concentration, and it decreased rapidly near the nozzle tip. On the other hand, it can be seen that more small droplets are distributed for DME/diesel blended fuels than that for pure diesel fuel. From the results, it can be concluded that it is a good way to use DME addition into diesel fuel to enhance the fuel/air mixture formation.The homogeneous charge compression ignition (HCCI) combustion operated with DME and diesel fuel blend was investigated on a modified single cylinder diesel engine. The experimental work was carried out by using port fuel injection with pintle nozzle. The effects of fuel injection pressure, inlet air preheating and CO₂ addition in intake air on engine performance and emission characteristics were studied. The results show that port injection of diesel fuel with higher volatility DME with distillation temperature lower than ambient air temperature has the potential to improve both mixture formation and homogeneity as a result of DME flash boiling. It is also an effective way to control HCCI combustion and extend the engine operating range. Different proportion of CO₂ addition in intake air allows further control of combustion timing and engine load. When the proportion of CO₂ addition was increased from zero to 30%, the Brake Mean Effective Pressure (BMEP) was extended from 0.32MPa to 0.5MPa, and simultaneous reduction of NO_x and soot was achieved while HC and CO emissions slightly increased.A new chemical kinetic model of DME/n-heptane blended fuel is obtained, which includes a sub-model for NO_x. The simulation results of DME/n-heptane blended fuel HCCI combustion agree with the experimental results very well. In order to better understanding the detail information of DME/n-heptane blended fuel HCCI combustion characteristics, the effects of the intake temperature, compression ratio, equivalent ratio, engine speed and intake air pressure on the blended fuel HCCI combustion characteristics are investigated by CHEMKIN software. It reveals that both the low temperature reaction and high temperature reaction can be controlled by intake temperature, compression ratio, equivalent ratio and engine speed. Furthermore, CO₂ as an inter gas is added into the intake manifold. These results show that CO₂ can delay the production of OH free radical, which is very important for the low temperature reaction. The intermediate of HO₂,H₂O₂ and HCHO are the key factors for the high temperature reaction. Because CO₂ addition can reduce the combustion temperature, which inhibits these intermediate productions and controls the combustion process. In addition, the numerical results show that the more CO₂ concentration, the lower NO_x at high load conditions, but higher HC and CO emissions.